1. Field of the Invention
The disclosure relates to a constant current driving circuit and a light emitting diode (“LED”) backlight apparatus using the constant current driving circuit.
2. Description of the Related Art
In general, a conventional constant current driving circuit is configured to detect current flowing through a load circuit in real time. For instance, the conventional constant current driving circuit is configured as shown in FIGS. 11 and 12. The conventional constant current driving circuit performs basic operations as follows.
(1) A current detector RR is connected to a light emitting diode (“LED”) string including N number of LEDs in series to measure a voltage Vsen.
(2) The constant current driving circuit controls an output voltage Vout to allow the detected voltage Vsen to become equal to a target value REF and controls an output current Iout to be constant. The output voltage Vout and the output current Iout are controlled by controlling a gate voltage applied to a switching device SI. Referring to FIG. 13, the output voltage Vout and the output current Iout are controlled by adjusting a ratio of a turn-on time period Ton to a turn-off time period Toff.
The conventional constant current driving circuit needs to detect the voltage Vsen of the current detector RR regardless of whether the switching device SI is turned on or turned off. However, since the current detector RR is connected to the load circuit in series, power loss in the current detector RR increases when the output current Tout increases.
In addition, when the current detector RR is connected to a high voltage side of the load circuit, a control integrated circuit (“IC”) is required to have a high endurance to a high voltage. On the other hand, in a case where the current detector RR is connected to a low voltage side of the load circuit, the load circuit is not grounded directly at one terminal thereof.
In addition, since a signal from the current detector RR is transmitted to the control IC, the load circuit does not needed to be provided with an input pin at an output terminal thereof.
Japanese Patent Publication No. 2010-040509 (hereinafter, “patent document”) discloses a conventional constant current driving circuit having a circuit configuration different than those as shown in FIGS. 11 and 12. The driving circuit disclosed in the patent document includes a current detecting resistor RISEN_i connected to the switching device in series and an inductance coil connected to the load circuit to perform basic operations as follows.
(1) A multiplier multiplies a reference signal REF, which is a desired value of an LED current, by an on/off control signal (e.g., a pulse width modulation (“PWM”) signal) of the switching device SI.
(2) The current detecting resistor RISEN_i detects a monitor signal ISEN_i. Since the current detecting resistor RISEN_i is connected to the switching device SI in series, the monitor signal ISEN_i is equal to the current flowing through the switching device SI. The monitor signal ISEN_i is calculated based on the following Equations 1 and 2. A differential amplifier compares a result of Equation 1 with the monitor signal ISEN_i to obtain a control signal PWM_i.Iled=ISEN—i/PWM—i  [Equation 1]ISEN—i=REF×PWM—i  [Equation 2]
FIG. 14 shows a relationship between the LED current Iled, the monitor signal ISEN_i, and the PWM signal PWM_i in the conventional constant current driving circuit disclosed in the patent document.
(3) The control signal PWM_i is feedback controlled such that the result of Equation 2, i.e., “REF×PWM_i,” becomes equal to the monitor signal ISEN_i. In other words, the control signal PWM_i is feedback controlled to allow the LED current Iled to become equal to the desired value of the LED current.
The reference signal REF, the monitor signal ISEN_i, and current IL in the inductance coil have waveforms, which are respectively shown in FIG. 9 of the patent document.
However, since the driving circuit disclosed in the patent document requires the multiplier, a circuit configuration of a switching balance controller becomes complex. In addition, the differential amplifier generates the control signal PWM_i based on the monitor signal ISEN_i, which is obtained by using the control signal PWM_i. That is, according to the driving circuit disclosed in the patent document, a convergence defect exists in feedback control of the control signal PWM_i.